The present invention relates to range changing in a measuring device, particularly to the switching of an impedance into or out of a parallel network of range impedances.
It is common to use a differential amplifier to drive a load at a particular voltage. The particular voltage to be maintained is connected to the noninverting input of the amplifier and the voltage developed across the load is fed to the inverting input of the amplifier. This feedback results in the voltage across the load being the same as the voltage at the noninverting input.
To measure current through the load, a resistor or other impedance, is added between the amplifier output and the load. The current to the load flows through this resistor resulting in a voltage across the resistor that corresponds to the load current. An appropriate voltage measuring device across the resistor then provides an indication of the load current.
Because of the feedback, the amplifier tries to maintain the load at the desired voltage independently of any voltage developed across the resistor.
In order to measure a wide range of currents, it is common to provide several different values of these current-sensing resistors, all switchable in parallel between the amplifier and the load, that is, between the two legs of a parallel range-selecting network.
As one of these resistors is switched into or out of this parallel range-selecting network using a relay or other mechanical switch, the voltage across the network changes. This often results in a sudden change in the voltage across the load. Because of the feedback, the output voltage quickly returns to the desired voltage. However, because the response time of the circuit is finite, a voltage spike will occur at the output.
It is possible to use an FET to switch the resistor into or out of the network by connecting the resistor in series with the drain and source of the FET across the network. A ramp voltage with respect to the source applied to the FET gate causes the switching to take place at a slower rate than a relay, resulting in less of a voltage spike or glitch at the output.
Unfortunately, the switching characteristics of the FET are substantially different depending on the polarity of the signal being switched.
For example, in the case of an n-channel FET, when the drain is positive with respect to the source, the FET very quickly turns on or off when the ramp is in the vicinity of the pinch-off voltage. On the other hand, when the drain is negative with respect to the source, the switching begins when the ramp is roughly at the pinch-off voltage plus the drain to source voltage and ends near zero volts (or vice versa). In this case, the FET output basically follows the ramp between zero and the pinch-off voltage plus the drain to source voltage. This polarity configuration can be described as operating the FET in follower mode.
Similarly, complementary results occur in the case of a p-channel FET.
It is of course desirable to prevent glitches when switching between any of the ranges. However, it significantly complicates the circuit design to provide an FET switch for each resistor in the network. This is particularly true in very low current applications where relays in addition to the FETs are required for proper isolation.